The present invention relates to a method for separating layers from articles provided with a hard material layer comprising a Ti compound, in an alkaline solution comprising
hydrogen peroxide
at least one base
at least one acid and/or a salt of an acid.
DD 228 977 describes a method for separating of TiN layers, in particular for separating TiN layers applied onto nickel substrates. The articles to be treated are therein placed into a hydrogen peroxide solution with a content of 35 Ma % peroxide for approximately 3 minutes at a temperature of approximately 70xc2x0 C. to 80xc2x0 C., subsequently rinsed in water, dried and then mull-wiped off. For the separating of TiAlN layers, this method is unsuitable, since TiAlN is poorly soluble in hydrogen peroxide solution.
According to GB 2 127 042 titanium nitride hard material layers on substrates of stainless steel are separated in aqueous nitric acid at temperatures above 70xc2x0 C. The layer separation time for a 1 xcexcm thick layer at 70xc2x0 C. is approximately 50 hours. This extremely long layer separation time is of great disadvantage.
A method for the layer separation of hard material layers on different metal substrates is known from U.S. Pat. No. 4,746,369. The acidic layer separation bath is composed of hydrogen peroxide as oxidant and a phosphoric acid or nitric acid. Further, different surface-active materials are used. The very low pH value of the solution of less than 0.5 is of disadvantage for the application on high-speed steel-substrate-HSS (High Speed Steel).
DE 41 10 595 proposes to carry out hard material layer [separation] from tool surfaces in a hydrogen peroxide solution stabilized by complexing agents with potassium sodium tartrate ttrahydrate or sodium gluconate being used as the complexing agent. On the one hand, TiAlN layers are thereby not separated at a satisfactory rate and, on the other hand, the stabilizers used exhibit only a conditionally stabilizing effect.
From DE 41 01 843 a method is known for the layer separating of objects layered with hard material layers, wherein the objects are treated with a solution comprising tetrasodium diphosphate and hydrogen peroxide. Typical are hydrogen peroxide concentrations of 8 to 12%, the relatively high tetrasodium diphosphate concentration of 8 to 12%, high process temperature at boiling temperature, and high pH values of 8 to 12. If the solution is concentrated by evaporationxe2x80x94which occurs relatively rapidly at the high temperaturesxe2x80x94phosphatization of the already delayered surface can occur. Titanium nitride and/or titanium nitride/carbide hard material layers are separated.
Lastly, from DE 43 39 502 methods are known which are specific to the layer material, for the layer separating of metal substrates, in particular also of hard metal substrates, which comprise hard material layers comprised of either TiN, TiCN or TiAlN or of layer systems comprising TiN/TiAlN.
Therein an alkaline solution with hydrogen peroxide, at least one base as well as at least one salt of mono- and dicarboxylic acids is used.
Depending on the layer to be separated, and while protecting the substrate, the solution bath is composed of a multiplicity of components, specific to the layer material.
Building on the last-cited method specific to the layer material, it is the task of the present invention to propose a method with which significantly simpler and more flexibly large-scale industrial high-speed steel substrates can be delayered, and specifically independently of whether or not they are layered with a TiN, a TiCN and/or a TiAlN hard material layer. In contrast to the above described method, according to the invention one and the same method becomes possible for the separation of all cited hard material layers from HSS substrates.
Definitions
1) High-Speed Steel
A high-speed steel is characterized by high carbon concentrations of up to 1.5%, and additions of strongly carbide-forming elements such as chromium, molybdenum, tungsten and vanadium. Up to 12% of cobalt is comprised in some of the more complex grades.
This steel is referred to as high-speed steel because it retains its high hardness at high-speed applications (see D. T. Llewellyn, Steel: Metallurgy and Application, Butterworth-Heihemann Ltd., Oxford 1992, p. 174).
2) Phosphate
As a phosphate is defined a salt of a phosphoric acid, diphosphoric acid, triphosphoric acid, etc. For example, the phosphate xe2x80x9ccalcium phosphatexe2x80x9d is a salt of phosphoric acid, xe2x80x9cdisodium dihydrogen phosphatexe2x80x9d is a salt of diphosphoric acid, further xe2x80x9cpentasodium triphosphatexe2x80x9d is a salt of triphosphoric acid.
3) Phosphonate
By a phosphonate is understood a salt of a phosphonic acid, in particular of an organic phosphonic acid, i.e. of a phosphonic acid with organic substituents.
The posed task is solved thereby that for the layer separation of HSS substrates with at least one layer comprising TiN, TiCN or TiAlN, at least one substance from the group of phophates, phosphonates and phosphonic acids is used.
Preferably the following specific method guidances are used alternatively or in combination:
the phosphate disodium dihydrogen pyrophosphate, Na2H2P2O7 and/or pentasodium triphosphate (Na5P3O10) are preferably used;
as the phosphonic acid is preferably used aminotri(methylene phosphonic acid) and/or 1-hydroxyethane (1, 1-diphosphonic acid). These phosphonic acids are added to the solution as such. This applies generally to the use of phosphonates, phosphates and phosphonic acids.
It has been found, at least preliminarily, that the use of 1-hydroxy ethane(1, 1-diphosphonic acid) is especially suitable.
The process temperature of the solution bath is maintained at 20xc2x0 C. to 80xc2x0 C. (the range limits are included), if appropriate by heating and/or cooling;
The peroxide concentration is s elected between 5 and 50 wt. % (the range limits are included);
The concentration of phosphate and/or phosphonate and/or phosphonic acid is selected between 0.1 and 10 wt. % (the range limits are included);
Relatively large solution bath volumes, for example xe2x89xa750 l or even xe2x89xa7100 1 can also be stably operated;
The solution comprises preferably distilled water, hydrogen peroxide, the cited base, for example NaOH, and the cited phosphates and/or phosphonates and/or phosphonic acids, at least by a far predominant fraction.
This permits circumventing a layer material-specific and complex solution composition and the feasibility is created of separating layers of each of the listed hard material layers from HSS substrates with the same solution. Furthermore, the extremely high apparatus expenditure is dispensed with, which must be expended if an alkaline hydrogen peroxide bath with a pH value of 8 to 12 is operated in the boiling temperature range. Due to the low process temperature at which the solution bath according to the invention can be operated, the danger of phosphatization is also absent, which exists at high process temperatures in the boiling temperature range due to rapid concentration through evaporation.
This offers the capability for economic and large-scale industrial layer separation to operate stably delayering baths with large volumes, for example of more than 50 l or even of more than 100 l . Therewith large quantities of hard-material layered HSS substrates, such as for example tools, can be delayered within a few hours.
In contrast to TiN and TiCN, TiAlN does not dissolve in hydrogen peroxide solutions even at increased temperatures. If the pH value of the hydrogen peroxide solution is raised by adding a base, such as NaOH, into the alkaline range with a pHxe2x89xa77, in addition to the cited TiN and TiCN layers, TiAlN layers are also dissolved. It was found according to the invention that this is also the case at low process temperatures.
An advantage of the alkaline hydrogen peroxide is the high process certainty with respect to corrosion of HSS material. In an alkaline medium HSS is highly inert. Thus, no corrosion danger for the HSS substrate exists even if relatively long layer separation times would have to be used.
The problem of instability of alkaline hydrogen peroxide solutions, in that an autooxidation of the hydrogen peroxide occurs which is catalyzed by metal ions which are released into the solution during the delayering process, which in turn leads to overheating and boiling-up of the delayering bath, is solved according to the invention through the addition of at least one phosphate and/or phosphonate and/or at least one phosphonic acid as stabilizer. Through this addition the layer separating is furthermore accelerated, and specifically with respect to all cited hard material layers. Through this addition the bath stability is improved such that the layer separation bath could be stably operated even at increased temperature. For maintaining the bath temperature, a heating and/or cooling system can be provided; if appropriate the bath temperature is regulated to a nominal value, with the heating and/or cooling system as the correcting element.
Regarding the differences in efficacy of phosphates and/or phosphonates and/or of phosphonic acid in the solution used according to the invention:
When using phosphate alone the bath service life is relatively modest, in the normal case it is between 1 and 3 charges, which, it is understood, is quite sufficient in certain cases. This relatively modest bath service life is due to the restricted stability of polyphosphates. The stability of polyphosphates in alkaline solutions is only moderate, in acidic solutions it is poor. Through hydrolysis the polyphosphates are converted to orthophosphates which are far less effective for the stabilization of the peroxide solutions.
In the case of organic phosphonates and phosphonic acids the behavior is different.
In comparison to the inorganic polyphosphates, phosphonates and phosphonic acids have very good hydrolytic stability. This can be traced back to the very high stability of the Cxe2x80x94P bonds of the phosphonates and phosphonic acids. In addition to the increased stabilizing effect, increase of the bath service life, phosphonates and phosphonic acids lead, furthermore, to a further acceleration of the layer separation reaction.